Stereophonic sound is essentially different from monophonic sound in that the sound image of a particular sound source is localized. If the sound source moves, the sound image is accordingly moved. Thus, the stereophonic sound gives a sense of space in expanse and in perspective, in which the sound image is faithfully localized.
The sound image in a monophonic sound does not move. Therefore, if the sound image of the monophonic sound is moved, the sound can resemble stereophonic sound.
The localization of the sound image depends largely on the difference between sound pressure levels of sounds coming out of right and left loudspeakers. Another factor is the difference in phase, that is, the difference between times taken by the sounds from both speakers to reach the listener. The sound image is localized at a position from which sound with a higher sound pressure level is produced and at a position from which a sound with an advanced phase is produced.
There has been proposed a system for producing simulated-stereophonic sounds whose sound images are not localized by changing the sound level and phase.
FIGS. 9a to 9g show various conventional simulated-stereo devices. Referring to FIG. 9a a monophonic input signal is branched at a point P and applied to respective left and right loudspeakers 3 and 4 through filters 1 and 2 having different frequency responses. The filters 1 and 2 are, for example, a high-pass filter and a low-pass filter, respectively, the output signals of which are shown in FIG. 9b. Alternatively, the filters 1 and 2 may be complementary comb filters which give output signals shown in FIG. 9c, or all-pass filters which are different in group delay time as shown in FIG. 9d.
FIG. 9e shows a simulated stereo device having a reverberation chamber 5 wherein a loudspeaker 6 is provided and a pair of microphones 7 and 8 are provided at different positions. Monophonic sound generated by the loudspeaker 6 is picked up by the microphones 7 and 8 and reproduced through loudspeakers 9 and 10.
In another device which is shown in FIG. 9f, each monophonic input signal branched at a point P is applied to respective adders 11 and 12. The monophonic input signal is further applied to a delay circuit 14 through a level controller 13 and branched at a point Q. One of the branched signal is applied to the adder 11, while the other branched signal is applied to the adder 12 through a phase inverter 15. The adders 11 and 12 are connected to loudspeakers 11a and 12a, respectively.
In a device shown in FIG. 9g, monophonic input signals branched at a point P are applied to filters 16 and 17 which have different characteristics from each other. The output characteristics of the filters 16 and 17 continuously change in accordance with the input signals branched from a point Q. Each of the output signals of the filters 16 and 17 are applied to respective loudspeakers 18 and 19 to be reproduced.
The simulated-stereo devices described above use either a level difference or a phase difference between sounds reproduced by the right and left loudspeakers. These methods are cited in "Spatial Hearing", Blauert, Morimoto, and Goto, 1988, Tokyo.
However, in the conventional simulated-stereo devices, the monophonic input signal is imparted with a similar stereophonic effect over the entire frequency range. Namely sound images of all sound source are moved. Therefore, when applied to a movie projector, all sorts of sounds are uniformly dispersed without localization in spite of the characteristic differences between the sounds in different frequency ranges, thereby giving a strange feeling to the audience, for example, in a case of a screen in which a singer sings a song, the position of the voice of the singer moves together with sounds of the background. This is very unnatural.